In a conventional two-stage gear reduction device, a ring gear of a first planetary gear train and a ring gear of a second planetary gear train are usually separately formed. In general, each ring gear has connection portions which are machined after the ring gear is formed, and the connection portions of the two ring gears are connected together via fasteners such as screws or pins. The ring gears fabricated in this manner require secondary machining and subsequent mechanical connection and thus have a high cost. In addition, eccentricity between the two ring gears may easily occur due to the fabrication or assembly process.